Wet strength resins are often added to paper and paperboard at the time of manufacture. In the absence of wet strength resins, paper normally retains only 3% to 5% of its strength after being wetted with water. However, paper made with wet strength resin generally retains at least 10%–50% of its strength when wet. Wet strength is useful in a wide variety of paper applications, some examples of which are toweling, milk and juice cartons, paper bags, and liner board for corrugated containers.
Dry strength is also a critical paper property, particularly in light of the recent trend for paper manufacturers to use high yield wood pulps in paper in order to achieve lower costs. These high yield wood pulps generally yield paper with significantly reduced strength when compared to paper made from highly refined pulps.
Wet strength resins also provide increased dry strength to paper.
Resins similar to those used for imparting strength to paper are also often used as creping adhesives. In the manufacture of some paper products such as facial tissue, bathroom tissue, or paper towels, the paper web is conventionally subjected to a creping process in order to give it desirable textural characteristics, such as softness and bulk. The creping process typically involves adhering the web, a cellulose web in the case of paper, to a rotating creping cylinder, such as the apparatus known as a Yankee dryer, and then dislodging the adhered web with a doctor blade. The impact of the web against the doctor blade ruptures some of the fiber-to-fiber bonds within the web and causes the web to wrinkle or pucker.
Polyamine-epihalohydrin resins, such as polyaminopolyamide-epihalohydrin resins often contain large quantities of epihalohydrin hydrolysis products. For example, commercial polyaminopolyamide-epichlorohydrin resins typically contain 0.5–10 wt % (dry basis) of the epichlorohydrin (epi) by-products, 1,3-dichloropropanol (1,3-DCP), 2,3-dichloropropanol (2,3-DCP) and 3-chloropropanediol (CPD). Epi by-products are also known as epi residuals. Production of such resins with reduced levels of epi by-products has been the subject of much investigation. Environmental pressures to produce resins with lower levels of adsorbable organic halogen (AOX) species have been increasing. “AOX” refers to the adsorbable organic halogen content of the resin, which can be determined by means of adsorption onto carbon. AOX includes epichlorohydrin (epi) and epi by-products (1,3-dichloropropanol, 2,3-dichloropropanol and 3-chloropropanediol) as well as organic halogen bound to the polymer backbone.
Several ways of reducing the quantities of epihalohydrin hydrolysis products have been devised. Reduction in the quantity of epihalohydrin used in the synthetic step is an alternative. Control over the manufacturing process is another option yielding compositions of reduced concentration of hydrolysis products. Treatment with nonpolymeric amine during the manufacturing process to yield compositions of reduced concentration of hydrolysis products is known. It is also known that chlorohydrin residues can be removed by adding both inorganic bases and amines. The chlorohydrin-removing steps are initiated after viscosity increase has taken place.
Post-synthesis treatments are also known. It is also known that epihalohydrin and epihalohydrin hydrolyzates can be reacted with bases to form chloride ion and polyhydric alcohols. Bases can be used during the synthetic step to reduce organo chlorine contents of wet strength composition to moderate levels (e.g., to moderate levels of from about 0.11 to about 0.16%) based on the weight of the composition. U.S. Pat. No. 5,019,606 teaches reacting wet strength compositions with an organic or inorganic base.
U.S. Pat. No. 5,256,727 teaches that reacting the epihalohydrin and its hydrolysis products with dibasic phosphate salts or alkanolamines in equimolar proportions converts the chlorinated organic compounds into non-chlorinated species. To do this, it is necessary to conduct a second reaction step for at least 3 hours, which adds significantly to costs and generates quantities of unwanted organic or inorganic materials in the wet strength composition. In compositions containing large amounts of epihalohydrin and epihalohydrin hydrolysis products (e.g., about 1–6% by weight of the composition), the amount of organic material formed is likewise present in undesirably large amounts.
Still further, U.S. Pat. No. 5,972,691 discloses the treatment of wet strength compositions with an inorganic base after the synthesis step (i.e., after the polymerization reaction to form the resin) has been completed and the resin has been stabilized at low pH, to reduce the organo halogen content of wet strength compositions (e.g., chlorinated hydrolysis products) to moderate levels (e.g., about 0.5% based on the weight of the composition). The composition so formed can then be treated with microorganisms or enzymes to economically produce wet strength compositions with very low levels of epihalohydrins and epihalohydrin hydrolysis products.
Other methods of treatment of the resins to reduce AOX content include treatments with carbon adsorbents, or ultrafiltration to produce polyaminoamide/epichlorohydrin resins with low AOX.
Halogenated by-products can be removed from products containing high levels of halogenated by-products as well as low levels of halogenated by-products by the use of basic ion exchange resins. However, there can be significant yield losses in wet strength composition and a reduction in wet strength effectiveness with this method.
It is known that nitrogen-free organohalogen-containing compounds can be converted to a relatively harmless substance. For example, 1,3-dichloro-2-propanol, 3-chloro-1,2-propanediol (also known as 3-chloropropanediol, 3-monochloropropanediol, monochloropropandiol, chloropropandiol, CPD, 3-CPD, MCPD and 3-MCPD) and epichlorohydrin have been treated with alkali to produce glycerol.
U.S. Pat. Nos. 5,470,742, 5,843,763 and 5,871,616, which are incorporated by reference herein in their entireties, disclose the use of microorganisms or enzymes derived from microorganisms to remove epihalohydrin and epihalohydrin hydrolysis products from wet strength compositions without reduction in wet strength effectiveness.
Moreover, U.S. Pat. No. 6,429,267 to Riehle, which is herein incorporated by reference in its entirety, discloses amongst other features, a process for reducing the AOX content of a starting water-soluble wet-strength resin comprising azetidinium ions and tertiary aminohalohydrin, which includes treating the resin in aqueous solution with base to form treated resin, wherein at least about 20 mole % of the tertiary aminohalohydrin present in the starting resin is converted into epoxide and the level of azetidinium ion is substantially unchanged, and the effectiveness of the treated resin in imparting wet strength is at least about as great as that of the starting wet-strength resin.
U.S. Pat. No. 6,554,961 to Riehle et. al. teaches an acid treatment. However the acid treatment of U.S. Pat. No. 6,554,961 is harsh in that it is conducted at a low pH and for a long period of time tending to degrade the molecular weight of to the polymer and decrease the reactive functionality resulting in lower wet strength efficiency. The acid treatment preferably is followed by a base treatment that rebuilt molecular weight and recovered wet strength efficiency. U.S. Pat. No. 6,554,961 taught acid treat first, followed by a base treatment.
U.S. Patent Application US 2003/0000667 A1, which is incorporated by reference in its entirety, is directed to high solids polyamine-epihalohydrin resin products, particularly polyamine-epihalohydrin resin products which can be stored with at least reduced formation of halogen containing residuals, such as CPD.
U.S. Pat. No. 6,222,006 describes thermosetting wet strength resins prepared from end-capped polyaminoamide polymers. The endcappers used are monocarboxylic acids or monofunctional carboxylic esters, and are used to control the molecular weight of the polyaminamide in order to obtain wet strength resins with a high solids content.
Each of the foregoing approaches has provided various results, and there has been a continuing need for improvement in the use of polyamine-epihalohydrin resin, especially at high solids content. In particular, there is still a need for resin compositions, such as wet strength, dry strength and creping agent resins, which can be provided in solutions or dispersion of reasonable viscosity at relatively high polymer solids concentrations. Thus, there is still a need for resins that can be prepared, stored, treated and transported as a dispersion or solution containing high solids concentrations without product deterioration from polymer crosslinking, such as gelation problems.